An example of virtualization technology of computers is a virtual machine. According to virtual machines, it is possible to mitigate limitations caused by the physical configuration of a computer. For example, by operating plural virtual machines in a single computer, the computer may be used as plural computers.
As a computer is virtualized by virtual machines, the storage unit of the computer (for example, an HDD (Hard Disk Drive)) may also be virtualized. Specifically, in the HDD of a computer, a file system is established for each virtual machine operating in the computer.
A file system of a virtual machine is merely a part of the physical HDD, and therefore snapshots of the file system may be easily saved. By saving snapshots, rollback may be easily performed when a failure occurs. Snapshots are basically saved in accordance with instructions input by a user (see, for example, Japanese Laid-Open Patent Publication No. 2005-332223). Furthermore, there are discussions of automatically saving snapshots at predetermined timings (see, for example, Japanese Laid-Open Patent Publication No. 2009-51452 and Japanese Laid-Open Patent Publication No. 09-146762).
However, as snapshots are saved, the storage capacity is consumed. Therefore, when many snapshots are needlessly saved, the storage capacity is wastefully consumed.